Emissions regulations for internal combustion engines have become more stringent over recent years. The regulated emissions of NOx and particulates from internal combustion engines are low enough that in many cases the emissions levels cannot be met with improved combustion technologies. Therefore, the use of aftertreatment systems on engines to reduce emissions is increasing. For reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NOx and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. The reductant is injected into a combustion engine's exhaust stream upstream of an SCR catalyst. In the presence of the SCR catalyst, the ammonia reacts with the NOx in the exhaust stream to reduce the NOx to less harmful emissions. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
For example, conventional reductant delivery systems can experience inherent delays and inaccuracies between a commanded dosing rate and the actual dosing rate due to the valve opening and closing characteristics associated with conventional pulsed urea dosage devices. Typical pulsed urea dosage devices pulse at high frequencies for high urea dosing rates and low frequencies for low urea dosing rates. When pulsing at high frequencies, the dosing valve is closed for short durations, but at low frequencies, the dosing valve is closed for long durations. Because the dosing valve is pulsed differently for different dosing rates, the actual dosing rate may be different than the commanded dosing rate.
Additionally, with many conventional SCR systems, the reductant dosing rate depends upon the real-time delivery of reductant into the exhaust stream. But, reductant dosers typically have relatively slow physical dynamics compared to other chemical injectors such as hydrocarbon injectors. Therefore, reliance on real-time reductant delivery may result in inaccuracies between the commanded reductant dosing rate and the actual reductant dosing rate during transient operating conditions due to the physical delays of the reductant dosing system. Generally, reductant dosing delays during transient operating conditions are greater at lower dosing rates and lesser at higher dosing rates. Accordingly, for smaller combustion engines that need smaller amounts of reductant dosing, the dosing system may experience undesirable delays during transient operating conditions. Such delays can cause the reductant delivery system to be out of phase with the NOx emissions rate, which can lead to undesirable NOx spikes at the tailpipe outlet.
Based on the above, a need exists for an SCR system that accounts for potential inaccuracies between commanded and actual reductant flow rates, as well as inherent delays associated with reductant delivery system dynamics, to reduce NOx emission spikes and improve the overall NOx conversion efficiency of the system.